Create PO Output Names from PrintDefinition.name Substring?

I want to create an expression for Print Organizer's Output Filenames using a substring of the PrintDefinition.name string. 

For example, if a sheet's PrintDefinition.name is: 003 - Typical Section Sheet I want to grab the first three characters of that string "003" and use that as part of the output file name. 

I did a little experimenting with the .NET string operators, but attempts like PrintDefinition.name.Substring(0,3) come up as invalid.

Any help would be most appreciated.

Thanks!